1. Field of the Invention
The present invention relates to an improved adjustable mechanism for mounting a seat for fore and aft movement. The invention has particular application in the mounting of motor vehicle seats although, while generally described in relation to that application, it is to be understood that the mechanism also can be used for mounting other types of seats.
2. Description of Related Art
An existing and well known type of adjustable, vehicle seat mounting mechanism has a fixed rail and a movable rail which is engaged with the fixed rail for movement therealong. The mechanism also includes a locking member which is mounted on one of the rails and which is pivotable between an engagement position in which it locks the movable rail relative to the fixed rail by projecting, usually laterally, through at least one pair of aligned apertures of the two rails, and a retracted position in which it frees the movable rail for adjustment along the fixed rail. As installed in a vehicle, the fixed rail of each of two laterally spaced mechanisms is secured to the substructure, pan or floor of the vehicle, with each movable rail secured to the underside of a seat of the vehicle. The locking members of the two mechanisms are inter-connected so as to be pivotable in unison, such as under the action of a release bar associated with one of the mechanisms.
While there are many forms of vehicle seat mounting mechanisms of that general type, examples are shown by U.S. Pat. Nos. 4,168,051 to Terada, 4,629,254 to Stolper et al and 4,958,799 to Clauw et al. Each of these patents illustrate a usual characteristic of known forms of mechanisms; namely, that they are formed from a metal plate, typically of steel, by roll forming, pressing and/or stamping. A further usual characteristic is that the rails are inter-engaged by a channel form of one receiving a channel form of the other. Often the channel form of the movable rail is simply slidable in or on the fixed rail, although U.S. Pat. No. 4,958,799 (Clauw et al) shows an arrangement in which bearing means are provided between the rails. A departure from the usual characteristic of rails formed from a metal plate is shown by International patent specification WO95/02520 (Clausen/Norsk Hydro A.S.). in which the rails are extruded, such as from an aluminium alloy.
There are design constraints on mechanisms having components conventionally formed from a metal plate. There is also difficulty in achieving a suitable balance between weight and strength. Vehicle manufacturers seek to minimise the weight of all vehicle components. However, the sectional thicknesses of the metal plate for the mechanisms can only be reduced to a certain limit. Therefore, there is a limit to which the weight can be reduced if strength is not to be compromised. Of course, a countervailing pressure for vehicle manufacturers is safety, and strength can not be adversely compromised if safety standards are to be satisfied. The proposal of WO95/02520 offers the possibility of weight saving, by utilising the more favorable weight to strength ratio of aluminium alloys. However, while extrusion of rails of an aluminium alloy obviates some of the design limitations inherent in processes for forming mechanisms from a metal plate, it introduces other limitations.